The syngeneic mixed lymphocyte reaction (SMLR) is the proliferative response of T lymphocytes cultured with syngeneic non-T lymphocytes. The ontogenic development of the SMLR has been studied in BALB/c mice. These results suggest that the impaired SMLR in very young mice is, at least in part, due to cell-mediated suppression of the SMLR. In studies reported above, we found that the SMLR reaches adult level of activity at 4 weeks of age in BALB/c mice. We now report that the SMLR declines with age in this strain. The decline was first documented at 12 months of age, when non-T spleen cells were less able to stimulate young adult T cells than were non-T cells from 2-\to 3-month-old mice. Finally, suppressor cells were demonstrated in spleen cell preparations from 24-month-old mice and may explain or contribute to the impaired SMLR in these animals. The methods used for the separation and the culture of mouse lymphocytes in the SMLR also routinely include FCS. In order to eliminate the possibility of stimulation with FCS, we have substituted 0.5 to 1.0% fresh syngeneic NMS for 10% FCS in the nylon wool separation of T and non-T cells and during the culture period. We found no difference in the level of 3H-Tdr incorporated on day 5 of the SMLR in the presence of 0.5 to 1.0% NMS or 10% FCS. In experiments on the generation of monoclonal antibodies to autoreactive T cells, four 6-week-old male BALB/c mice were immunized with activated cells from an SMLR and their spleen cells fused with the SP2/O-Ag 14. Three hybridoma antibodies were of the IgM class and one of the IgG class. In an indirect immunofluorescence test all four of the monoclonal antibodies raised to SMLR-activated T cells reacted with the majority (57 to 81%) of activated blasts from an SMLR and a small percentage (6 to 8%) of spleen cells. Significantly, these same antibodies did not react with alloactivated cells than with spleen cells. In future studies we will: (a)\determine whether the monoclonal antibodies to autoreactive T cells inhibit the autologous MLR and whether the inhibition is strain dependent; (b)\measure the frequency of autoreactive T cells in different mouse strains; and (c)\raise cloned lines of autoreactive and alloreactive T cells. (LB)